Daylighting is the use of natural light to reduce artificial lighting
in buildings1. Artificial lighting accounts
for up to 30% of the energy usage of typical office buildings. If you
include the additional cooling required by heat generated by artificial
lights, the total energy usage caused by artificial lighting could approach
50%. Using daylight to reduce the usage of artificial lighting can greatly
increase the energy efficiency of buildings.

This article will illustrate how Architects can use free and readily
available tools to integrate daylighting into their building design.

2.0 Assess Daylighting Potential

Daylighting requires illumination from exterior environment. The first
step is to determine the light levels outside the building. Daylight comes
from not just direct sunlight but also from illumination from the sky
on overcast days. Figure 1 illustrates the Design Sky illuminance levels
in different latitudes. Using the calculator on Autodesk Ecotect's website,
we can also determine the recommended Lux levels for a particular latitude.
The Design Sky illuminance levels "represent the horizontal illuminance
value that is exceeded 85% of the time between the hours of 9am and 5pm
throughout the working year. Thus they also represent a worst-case scenario
that you can design to and be sure your building will meet the desired
light levels at least 85% of the time."2

Once the outside light level is determined, we can calculate the Daylight
Factor to determine the feasibility of using daylighting in our design.
Daylight Factor is a ratio of outside illuminance over inside illuminance.
The Design Sky Illuminance Calculator in Figure 1 tells us that the outside
illuminance at latitude 37 is 8100 Lux. The design interior ambient light
level for office activity is 250 Lux. Table 1 tells us at the Daylight
Factor (DF) for this scenario is 3.09%. 3% is within the acceptable range
for daylit spaces. DF under 2% will feel gloomy, and DF over 5% could
result in a design that is over-glazed, resulting in too much heat gain
or heat lost.

The Glazing Factor (GF) Calculation (LEED version 2.2) is also a ratio
of interior illuminance to exterior illuminance under overcast sky conditions.
Unlike the basic Daylight Factor calculation, the Glazing Factor takes
into account the window and skylight size and configuration in its calculation.
Table 2 illustrates a hypothetical building with 5' high vision windows
and 18" daylight windows on the North and South elevations. On the East
and West elevations, 5' high vision windows cover 50% of the facade. Our
hypothetical building achieves a Glazing Factor of 2.53%. These simple
calculations show that daylighting can be employed without greatly compromising
the comfort and energy performance of the building.

3.0 Solar Shading to Control Glare and Heat Gain

Daylight design involves more than providing useful illumination in the
building. Often, it has to address the negative side effects of natural
light: glare and heat gain/loss. Glare comes from excessive contrast that
usually results from direct beam sunlight striking interior surfaces.
Unwanted heat gain occurs when sunlight enters the building on warm days.
The solution to glare and heat gain comes in some form of sun control
strategy that employs sun shades, light shelves, blinds, or shade cloth.

The sun control strategy to use depends on climate and orientation of
the facade. This article will study a building in the San Francisco Bay
Area (Latitude 37.6). At this latitude, the shading strategy can be summed
up as follows:

North Elevation: no shading required. Indirect sunlight makes this
facade ideal for daylighting.

East Elevation: low sun angles in the morning and no sun in the afternoon.
Active shading devices, blinds, or shade cloths works best for glare
control.

West Elevation: no sun in the morning but low sun angles in the afternoon.
Active shading devices, blinds, or shade cloths works best for glare
control. Afternoon heat gain is a problem in this climate, it is best
to reduce the size and amount of West facing windows.

South Elevation: high sun angles most of the day makes this facade
ideal for passive shading strategies.

FIGURE 2. SUN PATH DIAGRAM (SOUTH ELEVATION)

The most common passive shading strategy uses a combination of sunshade
and light shelf. Figure 2 shows the path of the sun from outside South
facing windows. We can use this diagram to optimize our shading strategy.
There are two reasons for shading: reduce heat gain and eliminate glare.
These two criteria are expressed as dashed lines in Figure 2. Each dashed
line expresses an angle above which the sun will be shaded.

Thermal shading control heat gain in the summer and allow sunlight to
warm the building in the winter. In Figure 2, the thermal shade angle
is set at 45 degrees. In the summer, this angle will shade the window
from 10am to 5pm. In the winter, this window will not be shaded at all.
Solar thermal shading works best for sunshades above vision windows (windows
between 2'-6" and 7'-6").

Glare shading prevents direct sunlight from striking interior surfaces,
creating excessive contrast. Figure 2 shows the glare shade angle at 25
degrees. At this angle, direct sunlight is shaded from 9am to 5pm in the
Spring, Summer, and Fall. In the Winter, direct sunlight is shaded from
10am to 2pm. Glare shading is usually used to control glare on daylight
(clerestory) windows, glazing above 7'-6". Daylight windows, when
used in conjunction with light shelves to shield direct sunlight from
the building perimeter, bounce light off the ceiling to light up the building
interior.

The thermal shading line and glare shading line can be adjusted to meet
the designer's criteria. We can balance the competing needs of more daylight
and less glare/heat gain. Where the windows are not adequately shaded,
users will usually deploy shade cloths to address thermal comfort and
glare issues. Shade cloths, with 3-15% openness, usually filter out most
of the useful daylight entering the building. Therefore, it is essential
to design the sunshades and light shelves to optimize the amount of daylit
hours to reduce the use of artificial lighting.

FIGURE 3. SECTION OF SUNSHADE & LIGHT SHELF

Figure 3 illustrates how the sunshade and light shelf could be designed
to meet criteria set in Figure 2. Sunshades devices outside the building.
In this illustration, the sunshade shields the vision window at a 45 degrees
angle, keeping out heat gain and glare most of the day in Summer and Fall.
Light shelves are inside buildings. They protect the building perimeter
from direct sunlight and bounce light off the ceiling to light up the
building interior. In this example, the light shelf is around 4 feet deep
to prevent glare up to a 25 degree sun angle. As you can see from this
illustration, the depth of the sunshade and light shelf can also be constrained
by construction and code issues. Cantilevered sunshades require structural
reinforcing to withstand wind uplift. Interior light shelves can obscure
fire sprinkler distribution.

4.0 Daylight Modeling

Computer software daylight modeling can be a very useful tool to analyze
and optimize the daylighting design. Many software programs are available;
most are free to use. The most commonly used software is the Unix-based
Radiance Sythethic Imaging System6 developed
by Lawrence Berkeley National Laboratory. Radiance is reputed to by very
accurate; however, it is not easy to use. The simulation in this study
was done using DIALux7, a professional lighting
design software widely used in Europe.

FIGURE 4. FLOOR PLAN

This study assumes a 60' wide rectangular building (Figure 4) in San
Francisco oriented along an East-West axis. There are continuous ribbon
windows along the North and South elevations from 3'-0" to 9'-0".
The ceiling height as modeled is a standard 10'-0".

FIGURE 5. BUILDING SECTION ALONG NORTH-SOUTH AXIS

Figure 5 is a section of the building across the North-South axis. The
North windows are unshaded. The South elevation has sunshades and light
shelves as shown in Figure 3.

FIGURE 6. ILLUMINANCE (LUX) ALONG NORTH-SOUTH AXIS (SUMMER)

Figure 6 depicts the illumination levels at three different times on
a typical Summer day. The daylight properties were set for June 21st,
the longest day of the year. Calculations were done for 9am, 12noon, and
5pm to gather data on daylight levels throughout the day. The simulation
shows that the recommended ambient light levels of 250 Lux was achieved
across the entire floor plate during the middle of the day. In the morning
and late afternoon, designed light levels were achieved within 15' of
the South windows and 10' from the North windows. It does appear that
light levels of 200 Lux was achieved across the entire floor from 9am
to 5pm. There could be significant energy savings from lighting in the
Summer.

FIGURE 7. ILLUMINANCE (LUX) ALONG NORTH-SOUTH AXIS (FALL)

Figure 7 illustrates illumination levels on a typical Fall day. Daylight
properties were set for September 21st (Equinox). In the middle of the
day, the design light level was achieved with daylight alone. The South
side of the building was able to achieve 200 Lux for most of the day.
The North side had light levels below 200 Lux at the begining and end
of the work day. It appears that there could be energy savings from lighting
in the Fall and Spring as well. This simulation shows that the design
could benefit from higher daylight windows and/or higher ceiling on the
North side of the building.

FIGURE 8. ILLUMINANCE (LUX) ALONG NORTH-SOUTH AXIS (WINTER)

Figure 8. illustrates illumination levels on December 21st, the shortest
day of the year. In the middle of the day, the design light level was
achieved across the entire floor plate. Within 5' of South elevation,
the illuminance level spiked up to 4400 Lux. There would be glare issues
if workstations were place in this perimeter zone. At 9am, there was significantly
high Lux levels on the South side of the building, indicated by the spikes
on the graph. These numbers suggests direct sunlight falling on the workplane.
During the morning, interior blinds or shade cloths would likely be deployed;
therefore, there would not be any useful daylight. There could be some
energy savings from daylighting in the Winter, especially during the middle
of the day; however, due to the shorter days and low sun angles, the savings
would not be significant.

FIGURE 9. DIALUX FALSE COLOR RENDERING OF BUILDING INTERIOR

DIALux has many output options to help you visualize and understand data
from your simulation. Output options include 3D rendering, False Color
rendering, Workplane Photometric chart, and Workplane Isolines. Figure
9 is an example of a False Color rendering. It shows a 3D perspective
view of the building interior with different colors assigned to preset
illuminance values (Lux).

FIGURE 10. DIALUX WORKPLANE PHOTOMETRIC CHART

Figure 10 shows a Workplane Photometric chart. The workplane is typically
set to 2'-6" above finish floor. The chart allows you to see the
illuminance levels at various preset intervals across the entire floor
plan.

5.0 Conclusion

The proliferation of many open-source software and freeware designed
for daylighting has lowered cost and skillset required for Architects
to use daylighting in their design process. This article has only scratched
the surface of tools one can use to analyze and simulate daylighting in
buildings. However, with simple tools such as Excel, AutoCAD, DIALux,
and many online resources, this study was able to assemble important data
that could be used to improve the daylight performance of the building.